Answer: Healthy lilacs are very cold and wind tolerant, so it is possible that the shrubs need to be rejuvenated. This is done right after bloom by removing the oldest stems at the base and then removing any new shoots that are smaller than a pencil in diameter. You can remove all of the big old stems at once or over a period of two or three years. The shrubs will also appreciate a top dressing of compost or well aged manure or complete fertilizer according to the label instructions in early spring.
It is also possible that your lilacs are suffering from a borer problem. In this case you will find stems that are entirely dead. The deadwood should be removed and destroyed. You might wish to consult with your county extension (866-7920) as to the most up to date control measures suggested for that problem, as well as to verify the cause of the dieback.
Finally, depending on the variety, you may also observe over time that your lilacs have alternate better and less good blooming years, similar to the way some apples have "off" and "on" years. There is nothing you can do about this except enjoy the good years.
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